Jeff Koons is the Energizer Bunny of contemporary art. He just keeps going and going, banging the drum of mass iconography, self-acceptance and, above all, a uniquely American optimism about the affirming and transcendent properties of art. His sunny central message -- that the banality of modern life is, after all, the life we have, and that we should make the most of it -- is amplified into a cumulative, all-embracing whoop of joy in a new retrospective at Chicago’s Museum of Contemporary Art.